The platelet-activating factor acetylhydrolase of mouse platelets

Abstract

Platelet-activating factor (PAF) acetylhydrolases are a family of distinct enzymes with the common property of hydrolyzing and inactivating PAF. It has been shown that the structure and the biochemical behavior of these enzymes depend on their cellular origin. We studied the PAF acetylhydrolase activity in mouse platelets in order to investigate the unusual response of these platelets to PAF. We found that mouse platelets contain a PAF acetylhydrolase with an apparent K m value of 0.8 μM, suggesting a very high affinity for PAF. Contrary to other normal mammalian cells and tissues, mouse platelet PAF acetylhydrolase is almost equally distributed in the membranes and the cytosol and is characterized by an extreme sensitivity to heating. The enzyme requires the presence of dithioerythritol for maximal activity, it is affected by 5,5′-dithiobis(2-nitrobenzoic acid) and N-ethylmaleimide, and it is strongly inhibited by phenylmethylsulfonylfluoride. We purified, to near homogeneity, the PAF acetylhydrolase from mouse platelet membranes and demonstrated that it is a protein relatively abundant in the membranes with an apparent molecular weight of 270 kDa. Electrophoretic analysis, under reducing conditions, revealed four bands and one duplet with molecular weights of 66, 55, 52, 49 and 62 kDa, respectively. Thus, PAF hydrolysis in mouse platelets is mediated by a PAF acetylhydrolase having biophysical and biochemical properties more intricate than those of the PAF acetylhydrolases found in other species.